A ground-based stationary gas turbine, such as a gas turbine comprises a compressor section for compressing air, a combustor section in which fuel with compressed air is burned, and a turbine section, in which the exhausted burned mixture of fuel and air is used to drive rotation of a turbine section, in order to generate mechanical energy, from which in particular also electrical energy may be generated. Such a gas turbine may require fuel supply control which may require knowledge of the fuel heating value, in particular “Lower Caloric Value” or “LCV”, and density of the fuel.
Most applications or operating conditions of gas turbines may have a relatively consistent or constant gas fuel supply, so these values may not change extraordinary. However, other applications or operating conditions of a gas turbine may comprise receiving different or various or varying gas fuels, changing between these various gas fuels rapidly. Thereby, the stability of the engine control and the running conditions and the limitation of the allowable engine operating envelope may be compromised.
In circumstances or operating conditions, where a consistent or constant fuel composition is expected as the supplied fuel, the LCV value and the specific gravity (“SG”) from which the mass density of the fuel may be calculated, may be provided by the user of the gas turbine who may have performed a fuel analysis, wherein this fuel analysis or its results may have been entered into the control system during commissioning. Thereafter, in a conventional system, the physical/chemical properties of the fuel are assumed to be constant, including assumptions of a constant LCV and SG.
EP 0 727 574 A1 discloses a method and device for regulating the NOx emission of an internal combustion engine, wherein it has been found that there is a relationship between the mechanical efficiency of a combustion engine and the combustion temperature and the combustion temperature is of importance for the NOx emission.
EP 1 387 073 A2 discloses charge density control for an internal combustion engine, wherein a value indicative of a desired charge density of a combustion mixture for the engine is determined as a function of an engine speed of the engine and a load on the engine.
US 2009/0192698 A1 discloses a method for automatically controlling a stationary gas engine, wherein an engine speed control deviation is computed from a set engine speed and an actual engine speed and a set torque is determined as a correcting variable from the speed control deviation by a speed controller, where a set volume flow is determined as a function of the set torque to establish a mixture throttle angle and a gas throttle angle, and wherein the set volume flow is varied to adjust the gas throttle angle by a correction factor.
U.S. Pat. No. 4,380,898 discloses a fuel control system for a gas turbine engine, wherein a trimmer device reduces the effect of variation of engine geometry in cold starts, wherein the trimmer uses a throttle lever angle and engine inlet pressure to compute the desired value of a thrust-related parameter.
EP 2 450 551 A2 discloses systems and methods for compensating fuel composition variations in a gas turbine, the method including monitoring at least one fuel parameter associated with a turbine combustor; monitoring one or more combustion dynamics characteristics associated with the turbine combustor; monitoring one or more performance and emissions characteristics associated with the turbine; estimating fuel composition based at least in part on the at least one fuel parameter, the one or more combustion dynamics characteristics, and the one or more performance and emissions characteristics, and adjusting at least one fuel parameter based at least in part on the estimated fuel composition.
If the fuel composition changes during operation on gas the governor will automatically adjust the fuel valve in order to maintain the closed loop control objectives (normally output shaft speed). However there are operational limits on the engine achieved by means of pre-defined boundaries of fuel heat input and, as the fuel system applies the fuel lower calorific value (LCV) and specific gravity (SG) when interpreting the demanded heat input, these boundaries effectively move if the fuel composition has changed. This could result in compromised transient operation such as compressor surge, or failure to accelerate the gas generator (“hanging”). A further problem could be caused by the governor gains in the PI controller becoming incorrect, potentially compromising operational stability.
There may be a need for a method and an arrangement of controlling a supply of a fuel to a combustor of a gas turbine comprising a compressor upstream of the combustor, wherein an at least partially not controllable variation in the composition or in the physical/chemical properties of the fuel is accounted for, in particular in order to enable a stable operation of the gas turbine.
The above need is satisfied by the subject-matter of the independent claims. The dependent claims specify particular embodiments of the present invention.